hh3 wifi speed dropping

Possible hh3 fault, what do those who know more than me think. Have had BT infinity 1 for 15 months now and my download speeds have been consistently 37/38 mbps when connected via ethernet cable. The download via wifi to my laptop and ipad air were 23 and 37 mbps respectively. Recently the wifi speed has been dropping to less than 2mbps. The only way to restore speed is reset the hh3. The speed will stay up for a few hours and then drop again. When this happens the speed via ethernet remains at full speed. I have tried numerous manual and auto selection of channels, the speed remains low. Reset is the only thing that fixes the problem. There have been no changes of equipment or movement in the house, or in the neighbouring properties. I would be grateful for any suggestions.